The present invention is directed to the sealing or closing of centrifuge tubes and, more particularly, to the sealing of a centrifuge tube which is substantially closed at both ends and having only a small access opening at one end from which a neck or stem protrudes carrying a filler passage to the tube.
The typical centrifuge tube has a generally uniform cylindrical shape with one end closed and the other end being completely open to receive the fluid sample to be subjected to centrifugation. After introduction of the fluid sample into the tube, it is usually necessary to provide a very tight closure or capping means over the open end of the tube during centrifugation. The closure or capping mechanism must be extremely tight, especially when the tube is used in a centrifuge rotor of the type known as a fixed-angle rotor in which the tube cavities are oriented at an acute angle or approach an angle of 0.degree. with respect to the spin axis of the rotor. The hydrostatic pressure within the tube becomes extremely great when the centrifuge is rotated at speeds of 20,000 r.p.m.'s or greater. Any leak, no matter how minute, eventually causes a liquid vacancy in the tube. Since centrifuge tubes are usually flexible and sometimes very pliable, any loss of liquid may cause the tube to collapse. This in turn may cause the tube opening to pull away from the capping mechanism further aggravating the problem.
In a recent development, Steven T. Nielsen developed a new type of centrifuge tube, which makes sealing of the tube and support thereof within a centrifuge much simpler and more reliable. A U.S. patent application Ser. No. 912,698 Pending Group 350 directed to such improved tube, assigned to Beckman Instruments, Inc., the assignee of the present invention, has been filed concurrently with the present application. The invention of the said Steven T. Nielsen was made prior to the present invention and nothing is herein claimed as our invention that is shown or described in the Nielsen application, which is to be regarded as prior art with respect to this application.
The Nielsen centrifuge tube is a substantially closed cylindrically shaped tube having a small access or filler opening in one end thereof. Protruding from the opening is a neck or stem enclosing a filler passage through which liquid is inserted into the tube. In a preferred form, the Nielsen centrifuge tube is in the general shape of a "hot dog" with a narrow stem protruding therefrom. The neck or stem, and preferably the whole tube, is formed of a fusible material, such as a polyallomer plastic or the like, which may be heat sealed to close the passage. When the passage is sealed, the centrifuge tube surrounds the liquid sample and there is no requirement for a tube capping mechanism across the opening of the tube in contact with the liquid sample.
After the tube is placed in a centrifuge, a support plug, conforming to the shape of the upper surface of the tube, may be placed over the tube to provide additional support for the upper end of the tube and seal to prevent the hydrostatic force of the liquid within the tube from blowing open or perforating the upper end of the tube.
One prerequisite to the proper use of the Nielsen type tube is that it must be properly sealed in order to make its use reliable. A simple pinch closure of the neck, heated to fusion temperature, will seal the passage so that it will withstand moderate pressure. However, this simple sealing method is unsatisfactory for reliable performance under the high hydrostatic pressure generated in an ultracentrifuge rotor, particularly in such rotors where the angle of the tube approaches 0.degree. with respect to the spin axis of the rotor. In such rotors the internal pressures at the tube neck are in excess of 1,000 psi.
In making the seal, it is also necessary to avoid significant heating of the sample within the tube which may be just adjacent the opening in the upper end of the tube. Typically centrifuge samples are formed of biological material which could be damaged by heat. And finally, since it is desirable to support the upper surface of the tube during centrifugation with a plug conforming to this surface, it is necessary to conform or mold the neck of each tube so that the upper surface is always the same general shape.